Drillable core perforating gun and method of utilizing the same

ABSTRACT

The present invention includes a perforating gun and method of using the same to perforate casing in a wellbore. The perforating gun includes a hollow carrier having a core. Mounted within the core are components including at least one charge and at least one plug for sealing the charge within the core. Preferably, a pair of end plugs are used to seal the ends of the carrier about the charge or charges. A string of tubing is used to position the perforating gun adjacent the casing to be perforated. The charge or charges are detonated creating perforation tunnels in the casing and surrounding wellbore. A drill is then used to drill out the core of the carrier to provide ready access through the carrier for other tools and fluid flow. The carrier is ideally of a stronger material than the core components and enhances drillability of the core by providing guidance as the drill removes the core components.

FIELD OF INVENTION

[0001] This invention relates to perforating guns, to drillable toolsused in completing wellbores and to methods of employing suchperforating guns and drillable tools.

BACKGROUND OF THE INVENTION

[0002] A perforating gun is a downhole tool that uses either bullets, ormore commonly, shaped explosive charges. The charges shoot high-velocitygases to blow holes, referred to as perforations, in the casing orliner, cement and adjacent producing formation of a well. A bulletperforating tool has several barrels from which are fired high-velocityprojectiles called bullets. The bullets may be fired simultaneously orindividually. Usually, the bullet perforating gun is firedelectronically from a well surface.

[0003] During the process of drilling and constructing wells, a fluidsystem commonly known as mud is used to provide various desirablequalities, including lubrication, transport of drill cuttings, holestability, and control of fluid loss to the adjacent subterraneanformation. The physical property that describes the capability of aporous subterranean formation to conduct fluid is called permeability. Amud system used during construction of a well normally reduces thepermeability of the formation adjacent to the wellbore. In addition, thewellbore is sometimes lined with a casing. Perforating guns are used tocreate perforation tunnels through the casing and the damaged orlocalized region of reduced permeability in the surrounding producingformation.

[0004] Perforating guns generally include an outer carrier supportinginternally arranged explosive charges. The explosive charges are shapedand directed to create perforation tunnels of a predetermined pattern.After this perforation operation, the portions of the perforating gunremaining in the wellbore must be disposed of to accommodate passage ofother tools and fluid flow.

[0005] Currently, there are numerous options for disposing ofperforating equipment following detonation. In a first option, theperforating equipment or guns are conveyed by temporary tubing, andretrieved after detonation. The formation pressure should beoverbalanced by fluid pressure to ease removal of the gun from thewellbore. Overbalanced means that the pressure within a drilling stringand casing is greater than the pressure in the surrounding productionformation. This option of inserting, firing, and then retrieving the gunis commonly known as the “shoot and pull” method. However, the step ofretrieving the gun is time consuming and thus expensive during welldrilling and completion.

[0006] In a second option, guns are conveyed by permanent tubing, anddropped into the well after detonation. To prevent the obstruction ofthe perforated interval in the casing following this process, additionalwell depth is required so the guns can fall into an unused portion ofthe wellbore. The additional length of a wellbore drilled for thispurpose is called a rathole. Drilling a rathole can be very expensivewhen perforating long intervals. This option is commonly known as the“shoot and drop” method.

[0007] In a third option, guns are conveyed by permanent tubing, andleft hanging in place adjacent to the perforated interval. This methodblocks access to the perforated interval for wellbore maintenance orperformance data acquisition. This option is commonly known as the“shoot and hang” method.

[0008] In all three of the above methods, hollow carriers are typicallyused which swell and deform when explosives contained therein aredetonated. The guns are designed small enough in diameter so that, evenin their swelled state, the guns can be moved within the casing orwellbore after the explosives detonate. This is considered desirable forfuture well utility, even when the shoot and hang method is being used.

[0009] In a fourth option, guns are conveyed by permanent tubing, andsubstantial portions of the guns are constructed of a material thatfragments after detonation. Most of the resulting debris falls into therathole. However, a smaller amount of rathole is required because thedebris is in small pieces and it packs more tightly into the availablespace. This option is commonly known as the “disintegrating gun” method.One disadvantage of this method is that a substantial portion of the gundebris fragments can flow to the surface and plug surface equipment.Most of the larger debris fragments originate from a composite hollowcarrier section of the equipment.

[0010] In a fifth option, guns are conveyed by tubing or drill pipe, andthe metallic portions of the gun are made of aluminum. One disadvantageof this method is that most of the gun, including the carrier, is madeof aluminum, substantially increasing the cost of the gun's manufacture.The aluminum is relatively soft allowing the carrier to be completelydrilled away and discarded. Another disadvantage is that the gun istypically made of a smaller diameter and is thus not designed to be leftin the wellbore, and the smaller gun size limits the size of theperforating charges. This option is commonly known as the drillable gunoption.

[0011] In all five of the above methods, the explosive charges arecontained within the hollow carrier. The charges are held in place andaligned in a preferred orientation by attachment to a metallic chargestrip supported within the hollow carrier. After detonation, themetallic strip remains in the hollow carrier or falls into the rathole.The above methods all utilize pipe to convey the guns which allows longsections to be perforated simultaneously. Furthermore, this perforatingoperation can be done with substantially lower pressure inside thewellbore than in the formation adjacent to the wellbore. Consequently,mud does not flow into the surrounding production formation while theperforating is taking place due to the underbalanced condition.

[0012] In a sixth option, explosive charges are attached to the casingwhen it is installed in the well. These charges and a detonating cordthat connects the charges are subject to mechanical damage duringinstallation of the casing. Also, the charges and detonating cord areexposed to high temperatures within the wellbore. This exposure cancause damage if the charges and detonating cord are exposed to the hightemperatures for excessively long periods of time during the casinginstallation process.

[0013] In a seventh option, the guns are conveyed through the permanenttubing by a solid or stranded cable. This method is known as the“wireline” method. Guns conveyed in this manner can either be droppedinto a rathole or retrieved from the well. If retrieved, the guns mustbe retrieved in sections that are shorter than the available pressurecontrol equipment (commonly called a lubricator), or special connectorsmust be utilized between the sections of guns, or the formation pressuremust be overbalanced to allow safe removal. Dropping the guns oroverbalancing the formation pressure have the same disadvantagespreviously described. Recovery of the guns in sections through thelubricator is a time-consuming and expensive process when perforatinglong intervals. With this method, the length of the gun assembly issubstantially limited by the strength of the solid or stranded cable.Also, the charges are sometimes, but not always, contained within ahollow carrier.

[0014] In an eighth option, guns are conveyed through permanent tubingby a continuous reel of flexible tubing, commonly known as “coiledtubing”. This tubing is smaller than the permanent tubing that willremain in the well, and it can be run through the permanent tubing in aconcentric fashion. Guns conveyed in this manner can either be droppedinto rathole or retrieved from the well. If retrieved, they must beretrieved in sections that are shorter than the lubricator, or specialconnectors must be utilized between the sections, or the formationpressure must be overbalanced to allow safe removal. Dropping the gunsor overbalancing the formation pressure have the same disadvantagespreviously described. Recovery of the guns in sections through thelubricator is a time-consuming and expensive process when perforatinglong intervals. This method is known as the “coiled-tubing” method.

[0015] There are also additional permutations that combine variousfeatures of some of the different methods described above. Theseconventional methods of perforating casing and formation adjacent thecasing have numerous shortcomings. Therefore, there is a need forperforating apparatus and methods which have distinct advantages forcost and well performance as compared to conventional perforating gunsand methods.

SUMMARY OF THE INVENTION

[0016] It is an object of the present invention to provide a perforatinggun which perforates casing in a wellbore and which has a permanentcarrier with a core, containing charges, which may be drilled out afterdetonation to remove all internal components and to provide a relativelysmooth, clean core through which tools and fluid can readily pass.

[0017] A further object is to provide a method of perforating a casingin a wellbore of a production well which uses the aforementionedperforating gun with drillable internal components mounted within thecore of a permanent carrier, the carrier being positioned and ideallypermanently disposed with the casing.

[0018] This invention includes a perforating gun for perforating casingin a wellbore. The perforating gun comprises a carrier, at least onecharge and at least one plug. The carrier has a generally hollowinterior forming a core and is adapted to be mounted on a string oftubing. Ideally, there are a number of charges mounted on a carrierstrip which is sealed in the hollow interior of the carrier between oneor more plugs.

[0019] Internal components disposed within the core are designed to bereadily drilled out from the core of the carrier. For example, the plugsmay be made of aluminum or a plastic. To assist in the boring out of thecore, the internal components are ideally made of a softer material thanthe carrier. Preferably, the carrier is made of a strong, rigidmaterial, such as steel, and is permanently located within theperforated casing. The carrier enhances the drillability of the corewhile leaving the carrier relatively intact and with a relatively smoothinner surface through which tools and fluid may pass.

[0020] A method is also described wherein the above perforating gun maybe used to perforate a casing in a wellbore. The carrier of theperforating gun is then drilled out to provide a relative smoothinterior such that tools and fluid may readily pass there through. Thegun is ideally mounted on tubing which is permanently mounted with thecasing and need not removed prior to beginning production.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a schematic illustration of a drillable core hollowcarrier perforating gun, which is made in accordance with the presentinvention, the gun being mounted on tubing; and

[0022] FIGS. 2A-C illustrate a schematic sectional view of theperforating gun shown in FIG. 1, along with a hollow carrier gun sectionand a ballistic charge end section.

BEST MODE FOR CARRYING OUT THE INVENTION

[0023]FIGS. 1 and 2 A-C schematically illustrate a drillable core hollowcarrier perforating gun 10. In FIG. 1, gun 10 is suspended from a stringof tubing or drill pipe 12. String of tubing 12 includes a landingnipple 16, and tubing 20 which may contain auxiliary equipment. Theauxiliary equipment (not shown) includes conventional components such asa shock sub, a firing head or other desired tools. The shock sub is usedas a dampening device to dampen the compression wave transmitted to thetubing 12 by the increase in pressure below the end of the perforatinggun 10, subsequent to the firing of perforating gun 10. The firing headinitiates the burning of a primer cord. Conventional methods ofactivating the firing may be used such as sending a pressure waved downmud disposed in tubing 12. Alternatively, a wire could be deployed intubing 12 to carry an electrical impulse to activate firing the firinghead.

[0024] A casing 22 is set in the bottom of a wellbore by use of ananchor 24. A packer 26 cooperates with tubing 12 to seal the annulusbetween the casing 22 and the tubing 12 and to isolate it from thepressure inside the tubing 12 and below the packer 26.

[0025] Gun 10 is shown in greater detail in FIGS. 2 A-C. Gun 10 includesan outer hollow carrier 40 and end caps 42. Hollow carrier 40 ispreferably cylindrical and has an inner diameter or core 44. Disposedwithin core 44 is a charge strip 46 which supports a plurality ofcharges 48. A detonating cord 50 is attached to charge strip 46. Aballistic charge transfer end section 51 is located at the bottom ofcarrier 40. The charges 48 are ideally arranged to create perforationtunnels 23 (FIG. 1) in carrier 40 and surrounding casing 22 in aplurality of radial directions with the perforation tunnels 23 alsobeing ideally longitudinally spaced apart. Prior to detonation, it ispreferred that perforating gun 10 be filled with a high pressure gas.Otherwise, pressure waves might cause a longitudinal split in hollowcarrier 40 if the compressibility of the fluid in gun 10 were otherwisetoo low.

[0026] All the internal components disposed within core 44 arespecifically designed to be readily drilled out of carrier 40 afterdetonation. That is, they are either frangible or else made of a soft orlow strength material. Ideally, the carrier 40 is made of a higherstrength or rigid material such as steel. Thus, a drill, such as ahydraulically powered drill, may be introduced into the upper end ofcore 44 and remnants of components within core 44 drilled out. Thedrilled out or cleaned out core 44 will then allow for easy access ofany tools to be subsequently used. Further, with the core 44 drilledout, fluid may readily pass through carrier 40.

[0027] End caps 42 may be made of soft aluminum which is readily drilledthrough. Alternatively, materials such as ceramics or plastics may beused to form end caps 42 and the ballistic charge transfer end section.While not shown, one skilled in art will appreciate some type of sealdevice, such as O-rings may be used to create the seal between core 44and end caps 42. It is also envisioned that charges 48 could beencapsulated in a single end cap or plug made of plastic such aspolyurethane or polyethylene or the like. Thus, only a single plug wouldbe needed for each gun section. Alternatively, a series of theencapsulated charges 48 could be used. A common characteristic ofwhatever components may be placed in core 44 is that these components bedesigned such that they can be readily removed when core 44 is drilledout.

[0028] By providing the strong, outer non-drillable carrier 40 and thesofter drillable internal components, the drilling operation will resultin a core 44 that is relatively smooth despite the swelling ordeformation caused by the detonation of charges 48. Accordingly, fluidor other tools passing through core 44 are not likely to get hung up. Ofcourse, how smooth the interior core 44 becomes depends on the relativediameter of the drill bit and the diameter of core 44.

[0029] As seen in FIG. 1, it is desirable to have a carrier 40 which hasa large outer diameter than that of the tubing 12 to which gun 10 ismounted. This permits the inner diameter of the carrier 40 to be atleast as large as the inner diameter of the tubing 12. Consequently,inner core 44, after drilling, will not impede the passage of tools orfluid also passing through tubing 12.

[0030] Other advantages the present invention provides over previousperforating guns, including the use of a carrier which is completelydrilled away such that there is no remaining hanging portion, include:

[0031] (1) there is no need to drill excessive rathole for dropping theguns;

[0032] (2) if coiled tubing is used to drill out the core, it can bedone in an underbalanced condition which results in minimal damage tothe producing formation, and there is no need to kill the well;

[0033] (3) there is no requirement that the hollow carrier be movedafter perforating, so less clearance is required, and larger diameterhollow carriers can be used within the same size casing than withpreviously known perforating guns; and

[0034] (4) because the hollow carrier 40 is fixed in place, it is easierto drill out the components within the core 44 than it would be if theentire carrier 40 was free to move as in the fifth option of the priorart described above wherein the carrier is drilled away.

[0035] In the preferred implementation, one or more of guns 10 areconveyed by the tubing 12 and are preferably permanently locatedadjacent to the interval to be perforated in casing 22. Charges 48 aredetonated with perforations tunnels 23 being formed in casing 22. Acoiled tubing and drill assembly (not shown) is run down through tubing12. Then end caps 42 and the other remaining internal components withinhollow carrier 40 are drilled out. Hollow carrier 40 remains affixed inplace within casing 22. This is similar to the approach used in theshoot and hang method. However, in the present invention, the carrier 40is drilled out with the carrier 40 serving as a guide for the drillingoperation which removes any remaining internal components in core 44.The drilling operation also serves to “smooth” out the inner diameter orcore of carrier 40, if the diameter of the core 44 is similar in size tothat of the tubing 12 to which the gun 10 is mounted. Because gun 10 ismounted on tubing 12, many guns 10 may be mounted in series such thathundreds or even thousands of feet of casing 22 can be perforated at asingle time. Use of wireline perforating guns often restricts the lengthof casing 12 which can be perforated in a single operation. Tubing 12and carrier 40 can be left in place after the perforating operation.

[0036] After the drilling operation, the charges 48 and other internalcomponents do not obstruct the perforated interval from access bymaintenance or other devices for three reasons. First, the internalcomponents of the equipment above and at the connections within thehollow carrier 40 are constructed of materials that are easily drilledup with a bit and hydraulic motor (not shown) conveyed by concentrictubing, hence the name, “drillable core”. Second, the charge strip 46 ismade of materials that fragment upon explosive detonation or are easilydrilled up with a bit. Third, perforating guns 10 does not obstruct theperforated interval because the hollow carrier 40 is made larger indiameter than those used with methods that employ dropped or discardedcarriers. Consequently, there is no requirement that the carrier 40 beretrieved or dropped at some future time. It is also within the scope ofthis invention that a plurality of guns 10 may be joined together toperforate an extended length of casing 22. Again, all of their internalcores 44 may be readily drilled away.

[0037] In an alternative implementation of this method, guns could bedropped into the well, but still include easily drillable componentsabove and within the hollow carrier. In both cases, wireline- orconcentric-tubing conveyed tools can be used to access the fullperforated interval after the drillable components have been removedfrom the gun. However, this second implementation is less preferred forthree reasons. The hollow carrier must remain small enough to allow itto fall, the drilling operation is easier if the carrier or carriersremain attached, and some alignment problems can occur when trying toenter the detached hollow carrier with maintenance devices.

[0038] While in the foregoing specification this invention has beendescribed in relation to certain preferred embodiments thereof, and manydetails have been set forth for purpose of illustration, it will beapparent to those skilled in the art that the invention is susceptibleto alteration and that certain other details described herein can varyconsiderably without departing from the basic principles of theinvention.

What is claimed is:
 1. A perforating gun for perforating casing in awellbore, the perforating gun comprising: a carrier having a generallyhollow interior forming a core; at least one charge mounted within thehollow interior; and a plug made of a soft, drillable material whichseals at least a portion of the at least one charge within the carrier;wherein the carrier remains intact after the at least charge hasdetonated and may serve as a guide to a drill to drill out the at leastone plug and at least one charge from the core.
 2. The perforating gunof claim 1 wherein: a pair of plugs cooperate to seal the at least onecharge in the carrier.
 3. The perforating gun of claim 1 wherein: thereare a plurality of charges disposed in the carrier.
 4. The perforatinggun of claim 1 further comprising: a charge strip for positioning the atleast one charge within the carrier.
 5. The perforating gun of claim 1wherein: the carrier is made of a material having a higher tensilestrength than the at least one plug.
 6. A method of perforating a casingin a well core, the method comprising the steps of: mounting aperforating gun on tubing and positioning the perforating gun in awell-bore adjacent to a casing to be perforated, the gun having a hollowcarrier defining a core and having at least one charge stored in thecore and having at least one plug which seals the charge within thecarrier; detonating the charge to perforate the adjacent casing; anddrilling out the core of the carrier to provide a drilled out passagewaythrough which other tools and fluid may readily pass.
 7. The method ofclaim 6 wherein: the drilling out leaves the core of carrier relativelysmooth such that other tools may pass there through without hanging upon the drilled out core.
 8. The method of claim 6 wherein: the plug ismade of material which is softer than the carrier.
 9. The method ofclaim 6 wherein: the perforating gun includes a charge strip upon whichthe at least one charge is mounted; and the step of drilling out thecore removes the charge strip from the carrier.
 10. The method of claim6 wherein: the perforating gun is mounted upon a string of hollow tubingand is placed adjacent the casing to be perforated; and the core of thecarrier, after the core is drilled out, is at least as large as theinterior of the hollow tubing to thereby provide uninhibited passage oftools through the hollow tubing and the drilled out core of the carrier.11. The method of claim 10 wherein: the carrier is generally cylindricaland the outer diameter of the carrier is larger than the outer peripheryof the tubing.
 12. The method of claim 6 wherein: at least a hundredfeet of casing are perforated in a single detonation of the gun.
 13. Themethod of claim 6 wherein: at a thousand feet of casing are perforatedin a single detonation of the gun.